It is illegal because it is not taxed. If the government is not making money out of it they aren't interested.
Instead of someone being able to have the freedom to grow a plant from seed and smoke it, they would instead have to expose themselves to drug dealers and crimals.
Its just a plant people should just get over it. On a different note and since this is a stock forum anyone interested in profiting from weed used in cosmetics or pharmaceuticals should have a look at mxc or mmj. I hold a descent amount of mxc as i think they have huge potential.